SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-07
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DANE V. Dukhovni
Internet-Draft Unaffiliated
Intended status: Standards Track W. Hardaker
Expires: August 18, 2014 Parsons
February 14, 2014
SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-07
Abstract
This memo describes a downgrade-resistant protocol for SMTP transport
security between Mail Transfer Agents (MTAs) based on the DNS-Based
Authentication of Named Entities (DANE) TLSA DNS record. Adoption of
this protocol enables an incremental transition of the Internet email
backbone to one using encrypted and authenticated Transport Layer
Security (TLS).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 18, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Dukhovni & Hardaker Expires August 18, 2014 [Page 1]
Internet-Draft SMTP security via opportunistic DANE TLS February 2014
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 5
1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 5
1.3.2. Insecure server name without DNSSEC . . . . . . . . . 6
1.3.3. Sender policy does not scale . . . . . . . . . . . . 7
1.3.4. Too many certificate authorities . . . . . . . . . . 7
2. Hardening (pre-DANE) Opportunistic TLS . . . . . . . . . . . 8
2.1. DNS errors, bogus and indeterminate responses . . . . . . 8
2.2. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1. MX resolution . . . . . . . . . . . . . . . . . . . . 13
2.2.2. Non-MX destinations . . . . . . . . . . . . . . . . . 14
2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 16
2.3. DANE authentication . . . . . . . . . . . . . . . . . . . 17
2.3.1. TLSA certificate usages . . . . . . . . . . . . . . . 18
2.3.2. Certificate matching . . . . . . . . . . . . . . . . 20
2.3.3. Digest algorithm agility . . . . . . . . . . . . . . 23
3. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 25
4. Operational Considerations . . . . . . . . . . . . . . . . . 25
4.1. Client Operational Considerations . . . . . . . . . . . . 25
4.2. Publisher Operational Considerations . . . . . . . . . . 25
5. Security Considerations . . . . . . . . . . . . . . . . . . . 26
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 26
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Normative References . . . . . . . . . . . . . . . . . . 27
8.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
This memo specifies a new connection security model for Message
Transfer Agents (MTAs). This model is motivated by key features of
inter-domain SMTP delivery, in particular the fact that the
destination server is selected indirectly via DNS Mail Exchange (MX)
records and that with MTA to MTA SMTP the use of TLS is generally
opportunistic.
We note that the SMTP protocol is also used between Message User
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